Network Working Group D. Thaler
Request for Comments: 2991 Microsoft
Category: Informational C. Hopps
NextHop Technologies
November 2000
Multipath Issues in Unicast and Multicast Next-Hop Selection
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2000). All Rights Reserved.
Abstract
Various routing protocols, including Open Shortest Path First (OSPF)
and Intermediate System to Intermediate System (ISIS), explicitly
allow "Equal-Cost Multipath" (ECMP) routing. Some router
implementations also allow equal-cost multipath usage with RIP and
other routing protocols. The effect of multipath routing on a
forwarder is that the forwarder potentially has several next-hops for
any given destination and must use some method to choose which next-
hop should be used for a given data packet.
1. Introduction
Various routing protocols, including OSPF and ISIS, explicitly allow
"Equal-Cost Multipath" routing. Some router implementations also
allow equal-cost multipath usage with RIP and other routing
protocols. Using equal-cost multipath means that if multiple equal-
cost routes to the same destination exist, they can be discovered and
used to provide load balancing among redundant paths.
The effect of multipath routing on a forwarder is that the forwarder
potentially has several next-hops for any given destination and must
use some method to choose which next-hop should be used for a given
data packet. This memo summarizes current practices, problems, and
solutions.
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RFC 2991 Multipath Issues November 20002. Concerns
Several router implementations allow multipath forwarding. This is
sometimes done naively via round-robin, where each packet matching a
given destination route is forwarded using the subsequent next-hop,
in a round-robin fashion. This does provide a form of load
balancing, but there are several problems with approaches such as
round-robin or random:
Variable Path MTU
Since each of the redundant paths may have a different MTU,
this means that the overall path MTU can change on a packet-
by-packet basis, negating the usefulness of path MTU discovery.
Variable Latencies
Since each of the redundant paths may have a different latency
involved, having packets take separate paths can cause packets
to always arrive out of order, increasing delivery latency and
buffering requirements.
Packet reordering causes TCP to believe that loss has taken
place when packets with higher sequence numbers arrive before
an earlier one. When three or more packets are received before
a "late" packet, TCP enters a mode called "fast-retransmit" [6]
which consumes extra bandwidth (which could potentially cause
more loss, decreasing throughput) as it attempts to
unnecessarily retransmit the delayed packet(s). Hence,
reordering can be detrimental to network performance.
Debugging
Common debugging utilities such as ping and traceroute are much
less reliable in the presence of multiple paths and may even
present completely wrong results.
In multicast routing, the problem with multiple paths is that
multicast routing protocols prevent loops and duplicates by
constructing a single tree to all receivers of the same group
address. Multicast routing protocols deployed today (DVMRP, PIM-DM,
PIM-SM) [2] construct shortest-path trees rooted at either the
source, or another router known as a Core or Rendezvous Point.
Hence, the way they ensure that duplicates will not arise is that a
given tree must use only a single next-hop towards the root of the
tree.
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RFC 2991 Multipath Issues November 20003. Requirements
In the remainder of this document, we will use the term "flow" to
represent the granularity at which the router keeps state (if at all)
for classes of traffic. The exact definition of a flow may depend on
the actual implementation. For example, a flow might be identified
solely by destination address, or it might be identified by (source
address, destination address, protocol id) triplet. Hence "flow" is
not necessarily synonymous with the term "microflow" as used in RFC2474 [7], which also includes port numbers. Indeed, including
transport-layer information in the next-hop selection process can
actually be problematic. For example, if packets are fragmented, the
transport-layer information may not be available in every packet.
Furthermore, having the choice of path depend on transport-layer
fields may negate the benefit of caching information such as MTU for
use in subsequent connections between the same endpoints.
All of the problems outlined in the previous section arise when
packets in the same unicast or multicast "flow" are split among
multiple paths. The natural solution is therefore to ensure that
packets for the same flow always use the same path.
Two additional features are desirable:
Minimal disruption
When multipath is used, meaning that multiple routes contribute
valid next-hops, the chances are higher of routes being added
and deleted from consideration than when only the "best" route
is used (in which case metric changes in alternate routes have
no effect on traffic paths). Since a higher number of routes
may actually be used for forwarding when multipath is in use,
the potential for packet reordering and packet loss due to
route flaps can be much greater than when not using multipath.
Hence, it is desirable to minimize the number of active flows
affected by the addition or deletion of another next-hop.
Fast implementation
The amount of additional computation required to forward a
packet should be small. For example, when doing round-robin,
this computation might consist of incrementing (modulo the
number of next-hops) a next-hop index.
4. Solutions
We now provide three possible methods for improving the performance
of multipath and then discuss their applicability to unicast and
multicast forwarding.
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RFC 2991 Multipath Issues November 2000
Modulo-N Hash
To select a next-hop from the list of N next-hops, the router
performs a modulo-N hash over the packet header fields that
identify a flow. This has the advantage of being fast, at the
expense of (N-1)/N of all flows changing paths whenever a
next-hop is added or removed.
Hash-Threshold
The router first selects a key by performing a hash over the
packet header fields that identify the flow. The N next-hops
have been assigned unique regions in the hash function's output
space. By comparing the hash value against region boundaries
the router can determine which region the hash value belongs to
and thus which next-hop to use. This method has the advantage
of only affecting flows near the region boundaries (or
thresholds) when next-hops are added or removed. For ECMP
hash-threshold's lookup can be done with a simple division
(hash_value / fixed_region_size). When a next-hop is added or
removed, between 1/4 and 1/2 of all flows change paths. An
analysis of this method can be found in [3].
Highest Random Weight (HRW)
The router computes a key for EACH next-hop by performing a
hash over the packet header fields that identify the flow, as
well as over the address of the next-hop. The router then
chooses the next-hop with the highest resulting key value [4].
This has the advantage of minimizing the number of flows
affected by a next-hop addition or deletion (only 1/N of them),
but is approximately N times as expensive as a modulo-N hash.
The applicability of these three alternatives depends on (at least)
two factors: whether the forwarder maintains per-flow state, and how
precious CPU is to a multipath forwarder.
Some routers may maintain per-flow state for reasons other than for
supporting multipath. For example, routers typically keep per-flow
state for multicast flows so that they can maintain the list of
interfaces to which packets in the flow should be copied.
If per-flow state is maintained in a multipath forwarder, then
computation of the next-hop can be done by the router at state
creation time. This entails no additional computations at packet
forwarding time compared with normal forwarding to a single next-hop,
since the next-hop is precomputed. In this case, any method can be
used, including round-robin, random, modulo-N, hash-threshold or HRW.
Hash functions such as modulo-N, hash-threshold and HRW are better if
the forwarder state may be deleted for any reason during the lifetime
of a flow since subsequent next-hop computations by the router will
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RFC 2991 Multipath Issues November 2000
always select the same path. This also improves the usefulness of
debugging utilities such as traceroute. Finally, to maximize the
stability of paths (and hence the usefulness of traceroute, etc.),
the use of HRW is recommended over the other methods mentioned
herein.
If per-flow state is not maintained by the forwarder, then using
multiple next-hops requires that the next-hop be calculated at packet
arrival time. When CPU is more precious than stability of flow
paths, hash-threshold is recommended over the other methods mentioned
herein.
4.1. Unicast Forwarding
Depending on the implementation, unicast forwarding may or may not
keep per-flow state. We recommend that where forwarder
implementations keep flow state, routers should use HRW at state
creation time (and next-hop deletion time) to select the next-hop,
and that forwarders without per-flow state use hash-threshold.
4.2. Multicast Forwarding
Today's multicast forwarding engines use a cache of forwarding
entries indexed by group (or group prefix) and source (or source
prefix). This means that today's multicast forwarder's always keep
per-flow state, although for some multicast routing protocols, the
"flow" may be fairly coarse (e.g., traffic from all sources to the
same destination). Since per-flow state is kept by the forwarder, it
is recommended that the router always use HRW to select the next-hop.
Routers using explicit-joining protocols such as PIM-SM [5] should
thus use the multipath information when determining to which neighbor
a join message should be sent. For example, when multiple next-hops
exist for a given Rendezvous Point (RP) toward which a (*,G) Join
should be sent, it is recommended that HRW be used to select the
next-hop to use for each group.
5. Applicability
The algorithms discussed above (except round-robin) all rely on some
form of hash function. Equal flow distribution is achieved when the
hash function is uniformly distributed. Since the commonly used hash
functions only become uniformly distributed when the number of inputs
is relatively large, these algorithms are more applicable to routers
used to route many flows, than in, for example, a small business
setting.
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RFC 2991 Multipath Issues November 20006. Redundant Parallel Links
A related problem occurs when multiple parallel links are used
between the same pair of routers. A common solution is to bundle the
two links together into a "super"-link when is then used for routing.
For multicast forwarding, this results in the two links being reduced
to a single next-hop (over the combined link) which can be used to
prevent duplicates. When a unicast or multicast packet is queued to
the combined link, some method, such as those discussed earlier, is
still required to determine the physical link on which to transmit
the packet. If the parallel links are identical, then most of the
concerns discussed in this document are avoided with the combined
link. The exception is packet reordering, which can still occur with
round-robin, adversely affecting TCP.
7. Security Considerations
This document discusses issues with various methods of choosing a
next-hop from among multiple valid next-hops. As such, it does not
directly impact the security of the Internet infrastructure or its
applications.
One issue that is worth mentioning, however, is that when next-hop
selection is predictable, an attacker can synthesize traffic that
will all hash the same, making it possible to launch a denial-of-
service attack that overloads a particular path. Since a special
case of this is when the same (single) next-hop is always selected,
such an attack is easiest when multipath is not being used.
Introducing multipath routing can make such an attack more difficult;
the more unpredictable the hash is, the harder it becomes to conduct
a denial-of-service attack against any single link.
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RFC 2991 Multipath Issues November 200010. Full Copyright Statement
Copyright (C) The Internet Society (2000). All Rights Reserved.
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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